In a computer room, an IDC (Internet Data Center), or the like, an amount of heat generated from an electronic apparatus, such as a server, is increasing due to increasing integration density of the electronic apparatus. For this reason, it is desirable to appropriately cool the electronic apparatus.
Methods of cooling the electronic apparatus include a method that controls an indoor air flow to prevent excessive generation of heat accumulation, a method that cools the room by air conditioning, or the like, for example. In order to stably control the cooling state inside the room according to such cooling methods, it is desirable to measure the room temperature at a plurality of measuring points. When a three-dimensional temperature distribution inside the room can be obtained, this temperature distribution may be used to control the air conditioning or the like, so that it is possible to stably control the cooling state inside the room.
An example of the method that measures the temperature distribution includes a method that uses an optical fiber as a temperature sensor. Generally, in the case of the temperature measuring method that uses the optical fiber, measuring the temperature distribution with a high accuracy in a relatively narrow range, such as the server within the data center, is more difficult compared to measuring the temperature distribution with the high accuracy in a relatively wide range.
In a multipoint temperature measuring system using the optical fiber, optical pulses having a predetermined wavelength propagate through the optical fiber, and the temperature distribution along a propagating direction of the optical pulses is obtained from a variation with time of back-scattering light (that is, Raman scattering light) caused by the propagating optical pulses. In the temperature measuring method using the optical fiber, a distance resolution depends on an optical pulse width. In addition, an SNR (Signal-to-Noise Ratio) is determined based on a pulse area, and an elapsed time of temperature data at each measuring point for computing the temperature distribution from the variation with time of the back-scattering light. For this reason, in order to obtain a high distance resolution and a high SNR, a peak value of the optical pulses is desirably high. However, when optical pulses having a high laser power exceeding a threshold value (that is, a Raman threshold value) are input to the optical fiber that is a nonlinear medium, the so-called SRS (Stimulated Raman Scattering) occurs. Hence, the laser power is desirably suppressed to the threshold value or lower.
A laser light source is an example of a light source that emits the optical pulses. Generally, one of two kinds, namely, a solid state laser and a semiconductor laser, is used for the laser light source. The solid state laser is suited for high-precision measurement because of the wavelength accuracy and the peak value of the optical pulses are both high and noise is uneasily generated, however, the solid state laser is relatively expensive. On the other hand, the semiconductor laser is inexpensive compared to the solid state laser, however, the wavelength accuracy and the peak value of the optical pulses are both low and noise is easily generated. In addition, the SNR tends to deteriorate in the case of the semiconductor laser, because wavelength dispersion increases particularly at a far end. Accordingly, it is desirable to reduce the noise in the case in which the inexpensive semiconductor laser is used for the laser light source. There is a known method that improves the SNR by applying the Golay code, so as to use multipulses instead of a single pulse for the optical pulses. However, even when the multipulses are used for the optical pulses, there are cases in which the temperature accuracy required by big data analysis or the like, for example, cannot be satisfied.
In order to reduce the noise and further improve the SNR, it is conceivable to increase an accumulation time (or accumulation number) of the temperature data. The noise can be represented by a standard deviation in a predetermined temperature region, and is proportional to the accumulation time to the power −½. However, when the accumulation time of the temperature data at each of the measuring points increases, a time interval of the temperature measurement required until the temperature measured at each measuring point is determined becomes longer. Hence, in order to reduce the noise without varying the time interval of the temperature measurement, Japanese Laid-Open Patent Publication No. 2-201133, for example, proposes a method that distinguishes the temperature change and the noise, based on a magnitude of a difference between the temperature data at different times, to vary the accumulation number for every measuring point. However, in a case in which the magnitude of the noise is approximately the same as or greater than the magnitude of the temperature change, it is difficult to distinguish the temperature change and the noise. In addition, when the temperature change and the noise cannot be distinguished from each other, it is difficult to reduce the noise.
An example of related art may include Japanese Laid-Open Patent Publication No. 7-243920, for example.
According to the conventional temperature measurement using the optical fiber, it is difficult to reduce the noise, because it is difficult to distinguish the temperature change and the noise.